X-ray apparatus



w. F. wEsTENDoRP Jan. 17, 1939.

.X-RAY APPARATUS Filed June 4,V 1937 agMod sssuivm v Inventov bln) n Wh e W. ,a ,o Ww WA 3% W.

Patented Jan. 17,1939

para orties.

X-RAY APPARATUS wiilemr. Westendorp, Schenectady, N. Y., asignor to General Electric Company, a corporation of New York Application June 4, 19377, Serial No. 146,442

3 Claims.

Thepresent invention relates to high voltage X-ray apparatus, and particularly lto improve-- ments vin energizing means therefor; While not limited thereto, the invention is especially useful inconnection with I-ray tubes adapted. to be operated at voltages materially in excess of 100,000 volts. l' l Itis an object of the invention 'to provide an yX-ray vapparatus of such inherent characteris- .tics that it draws substantially no apparent power beyond that actually consumed in the generation of X-rays.Y Inconnection with this ob-y ject I havel in mind the' ultimate end that the power source for the X-ray apparatus and all auxiliary equipment associatedl therewithl may be of small dimensions and economical construction.

It is a further and more particular object of the invention to provide in connection with X-ray apparatus van improved X-ray tube energizing meansA of such constructionthat the total current drawn by the apparatus from its power supply source shall contain substantially no inductive or capacitive components attributablel to the inherent characteristics of the energizing means.

VIt is a further object of the yinvention to provide a lvoltage transformer suitable for energizing an X-ray tube, in which the lagging or magnetizing current drawn from the supply source by thetransformer is substantially equal to and effectively vcancels the leading current drawn v from the source as a result of the inherent electrical` capacity of the transformer windings or of the X-ray apparatus as a whole.

t is a further object to provide X-ray apparatus in which the inductive and capacitive components of wattless power are inherently neutralized without the use of auxiliary circuit elements for accomplishing such neutralization. (By the term wattless power I mean the quantity, sometimes called apparent power, which Y is represented by the product of a voltage and an out-of-phase component of current produced by such voltage.)

, Ina preferred embodiment ofthe invention these objects are attained by providing in the magnetic circuit of the X-ray tube energizing transformer one or more high reluctance or nonmagnetic portions whereby the inductive current drawn by the transformer is increased to the same order of magnitude as the capacitive current which it also draws. f

The features of novelty which I desire to protect herein Will be pointedl outwith particularity inthe appended claims. The invention itself, to-

Yed as (Cl. Z50-987) gether with further objects and advantages thereof, will best be understood by reference to the foliowing description taken in connection with the drawing, in which Fig. l is a graphical representation useful -in explaining the invention; Fig. Y

2 is a sectional view of an X-ray apparatus cornprising a particular application ofthe invention;

Fig. 3 is a sectional view taken on line 3-3 of Fig. 2, Y and Fig. 4 represents diagrammatically the equivalent circuit -of the construction of Fig. 2. t

In connection with most electrical load devices which are supplied with power by means of voltage transformers, the wattless or apparent power drawn by the transformer itself is substantially negligible in comparison with the power drawn'.

by the load device. However, in connection with high voltage X-ray apparatus wherein the .transformer is required to supply a current'of only a few milliamperes at a voltage materially in ex- Y cess of 100,000 volts, this is not necessarily true, as will vbe shown in the following.

The components of wattless power which a transformer draws from a supply line consist pri.- marily of the component corresponding to the magnetizing current of the transformer, and that corresponding to charging current drawn as a lresultl of the secondary winding capacity to ground. The magnetizing'power is capable of being predetermined as matter of design and can be maintained at a relatively low value irrespec-` tive of the voltages involved. The capacitive or charging power, on the other hand, is not similarly controllable and varies with operating voltage according to the following relationships.

The capacitive power drawn from a supply source by a given transformer can be represent- E -1=E227rfc Z'lrfC where E is the source voltage, C is the effective capacitance of the transformer windings andrf is the operating frequency of the source. Thus, for a transformer of given size the capacitive power increases as the square of the voltage applied thereto.

ELSE

An additional factor is involved, however, in K ing this factor with the relationship previously discussed, itV may be stated that for transformersl which are similarrexcept for dimensional changes necessitated by variations in voltage rating, the capacitive apparent power will increase roughly as the third power of such rating.

The signicance of this statement is illus- Ytrated in Fig. 1in whichcurve A represents the variations of capacitive apparent power with increasing voltage rating. It will be seen that whereas the power may be relatively very small at voltages up to 100,000, it will increase very rapidly at voltages above that value and Will have' risen about 12,500 per cent at 500'kilovolts. The magnetizing apparent power, on the otherhand, will, for conventional'transformer designs, vary in the less drastic manner indicated by curve B.

The excess of capacitive powerwhich thus occursV at extremely high voltages is of considerable importance in connection with the operationV of Xeray apparatus. For example, in a particular apparatus employing a voltage transformer to energize an X-ray tube at a potential of aboutr500 kilovolts (peak), it was observed that the wattless power drawn from the power source was about 14 lrva.'(kilovolt amperes); This wattless power was capacitive in nature and was about 3.5 times as great as the useful power consumed `by the Xfray tube itself. Consequently, either a relatively expensive neutralizing circuit element in the form of a reactor had to be provided, or the current-carrying capacity of the power source and the auxiliary supervisory equipment associated Ytherewith had to be made several times as large as the magnitude of the useful load alone seemed to indicate. Y

'I'his'diiculty may be overcome in an extremely inexpensive manner in accordance with my present invention, by constructing the X-ray energizing means in such a way that the capacitive power1 drawn thereby is inherently canceled by the inductive or magnetizing power which it also draws. Specifically, this may be accomplished by including in the magnetic circuit of the X-ray tube supply transformer ahigh reluctance or non-magnetic portion adapted to increase the transformer magnetizing current to a value commensurate with that of its charging current.

VThe important features of the invention will probably be most easily understood by referring to the specific embodiment of Fig. 1, wherein I have shown a high voltage X-ray apparatusv of the general type disclosed and claimedin an application of E. E. Charlton and W. F. Westendorp, Serial No. 113,862, filed December 2, 1936. As illustrated, this apparatus comprises a metal tank or container l0 substantially filled with an insulating dielectric medium such, for example, as an insulating oil. Immersed in the dielectric medium there is arranged an X-ray tube, shown at the left-hand portion of the iigure, and a high l voltage transformer for energizing the same.

The X-ray tube comprises a sealed evacuated envelope having a portion projecting exteriorly of the metal containerl and another portion projecting internally thereof. The externally projecting envelope portion constitutes an anode and includes a target I3 of a refractory metal such as tungsten which is adapted to develop X-rays upon being bombarded with high voltage electrons. This target is provided With external cooling means such, for example, as a cooling coil I4. It is also positioned adjacent a window I5 which permits the escape of X-rays in a desired direction. Surrounding the target there is provided a sheath l0- of lead or other X-ray impermeable material, which is grounded to the metallic struc- Vorder of 500,000 volts.

therefor there is provided a transformer comprising a primary winding 28 and a high potential ture of the container I0 by being directly mechanically connected thereto. y

The portion of the X-ray tube within the casing I 0 includes a series of insulating envelopeV portions I8, an enclosed thermionic cathode I9,

Vand a series of intermediate electrodes 20, of which only two are shown. This portion of the tube is provided rwith a series of external electrostatic shields 2l, each shield being connected ,to its corresponding intermediate electrode and being so positioned' as to decrease the radial'electrostatio'.

stress in accordance with'the principles set forth in the copending application, Serial No. 113,862, above referred to. Between the shielding members 2lV and the X-ray envelope proper there is provided an insulating sleeve 2G adapted to serve as a mounting means for the shielding members.

The X-ray tube constructiondescribed in the foregoing is adapted to be energized at voltages materially in excess of 100,000'volts and on the As an engizing means secondary winding 29., The primary winding, which preferablycomprises a helix of copper ribbon, may be connected to an Vexternal source of potential (not shown) by means of terminal connections 30, led out throughthe wall of the metal container I0 in any desired vfashion. The secondary winding is grounded at one end to the metal container I0 by means of a connection 33 Vwhich may includev a current meter 34, and thus is electrically connected to the anode structure of the X-ray tube. The other or high potential end Yof the secondary coil is connected to the cathode I9 by means'ofcurrent supply wires 35. As shown, this connection is made within the confines of an electrostatic shielding member comprising a hollow metal toroid 30, such toroid being connected to the high potential terminal of the secondary winding. and being so arranged as to enclose the cathode end of the X-ray tube. Intermediate points of the secondary winding 29 are connected to the electrostatic shields 2| by means of conductors 31. The energizing transformer is provided With a magnetic circuit which comprises an iron core 33 having a portion extending within the concentrically arranged transformer windings and insulated from the primary winding `28 Vby means Aof an interposed insulating cylinder 39. The core 38 is preferably of lamcomprise a rectangular arrangement as illustrated, although other geometrical forms are obviously not precluded.

A transformer secondary winding such as that illustrated may, when operated at extremely high potentials, and at commercial frequencies, draw a relatively large charging current as a result of the capacity whichexists between the various turns ofthe winding and the grounded walls of the enclosing container I0 and between the shielding member 36 and such walls. In order to compensate or neutralize this charging current, I provide in the magnetic circuit of the transformer core 38 one or more high reluctance of non-magnetic portions. For example, such portions may comprise interposed air gaps or sections of nonmagnetic` metals or insulating materials. These interposed sections have the effect of increasing the reluctance of the magnetic circuit of the transformer and thereby increasing the transformer magnetizing current to a value commensurate with that of the charging current. Since and to position them at the meeting points of i the various legs ofthe transformer as indicated at 4I, 32, t5 and fifi respectively. This results in a simplified construction by eliminating the customary interleaving of the laminations of magnetic material. i

The dimensions of the non-magnetic portions depend upon the relative sizes of the other elements of the transformer and upon the operating conditions involved. For any particular case they may readily be determined, either mathematically or empirically, by one skilled in the art. For example, in one instance in which the Vtotal length of the magnetic path was about 10 feet and the average cross-sectional area of the path was about .25 square foot, it was determined that the total length o-f the non-magnetic portions should be about one-eighth of an inch. This deterrnnation refers, of course, to a particular X-ray apparatus construction (specifically that shown in Fig. 2 hereof) and would require some modification for different arrangements of the apparatus.

Instead of arranging the Vnon-magnetic portions at the corners of the transformer core as described in the foregoing, it is in some cases preferable to group such sections into a single high reluctance portion (for example, an air gap) arranged at the center of the transformer secondary coil, as indicated at 41 or to provide this in addition to the others. It is an advantage of such an arrangement that any straying of the magnetic rlux into the walls of the apparatus container and into other regions of the apparatus as a result of the high reluctance gap is substantially prevented. Furthermoraas an incident of this same factor, the increase in magnetizing current for a given length of gap will be a maximum for a gap arrangement such as is indicated at 41.

The results obtainable by the use of my invention are represented diagrammatically in Fig. 4 which shows an equivalent circuit for X-ray apparatus constructed as described in the foregoing. In this gure the X-ray tube is shown at 4S and the capacitance of the transformer windings is indicated as a condenser 50 arranged in shunt across the X-ray tube. Also in shunt across the X-ray tube and in parallel with the condenser 50 there is shown an inductance comprising a coil 5I and a magnetic core 52 for the same, said core comprising a magnetic circuit which is closed except for the provision therein of a non-magnetic gap 53. If the dimensions of this gap are properly selected (as previously explained herein) the current drawn by the inductance may be made equal to that drawn by the condenser. Since these two currents are opposite in sign, they will neutralize one another as far as external connections are concerned. As a result the power source 55 shown at the left-hand side of the figure will have to be capable of supplying only the current actually consumed by the X-ray tube 49. Furthermore, the metering and control equipment indicated diagrammatically at 56 need be only large enough to handle this relatively small current. v n

'drawn thereby is on the order of It is a further advantage of X-ray apparatus comprising inductive andV capacitive elements which are vbalanced or tuned as described in the foregoing that there is less distortion of the supply voltage as a result of the pulsating current taken by the X-ray tube. This is a result of the fact that the tuned circuit comprised by the parallel arrangement of inductive and capacitive elements provides an energy storage which is effective tol avoid such distortion. From a practical standpoint this avoidance of distortionhas the effect of reducing the inverse voltage to which the apparatus is subjected during the non-conductive periods of the X-ray tube, which voltage tends to be increased in proportion to the amount of distortion present.

While I have shown a particular embodiment of my invention, it will be understood that many modifications may be made by those skilled in the art without departing from the invention, and I aim by the appended claims to cover all such modifications as fall within the true spirit and scope of the foregoing disclosure.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. In combination, a metal container enclosing a quantity of di-electric medium, a high voltage X-ray tube immersed in said medium, a transformer also immersed in said medium and having the terminals of its high potential winding connected to the electrodes of the X-ray tube, the electrostatic capacity between such winding and the container being so great that the capacitive current drawn thereby is on the order of or greater than the load current drawn by the X-ray tube, and an iron core for said transformer having a high reluctance portion in its magnetic circuit whereby the inductive current drawn by the transformer is materially increased.

2. In combination, ametal container enclosing a quantity of dielectric medium, a high voltage X- ray tubeimmersed in said medium, a transformer for supplying said X-ray tube immersed in the medium and having one of its terminals at high potential with respect to the metal container, a shielding member associated with said terminal for modifying the electrostatic eld existing between the terminal and the wall of the casing, the electrostatic capacity between the walls of the container and said transformer and shielding member being so great that the capacitive current or greater than the load current drawn by the X-ray tube, and an iron core for said transformer having a non-magnetic portion in its magnetic circuit whereby the inductive current drawn by the transformer is materially increased.

3. In combination, a metal container containing a quantity of dielectric medium, an X-ray tube immersed in said medium and intended to be operated at a voltage materially in excess of 100,000 volts, a transformer comprising a primary winding and a secondary winding for energizing said X-ray tube at its intended operating voltage, and an iron core for the transformer having a portion thereof extending through the secondary Winding, said core comprising a non-magnetic section within the confines of the secondary winding for materially increasing the inductive current drawn by the transformer during operation.

WILLEM F. WESTENDORP. 

